Enterococcus faecalis is a normal inhabitant of the mammalian intestinal tract, and is not normally pathogenic in healthy humans. However, it has become a major nosocomial pathogen in compromised patients and it can cause a highly fatal form of endocarditis. The medical importance of the enterococci is enhanced by their high degree of inherent resistance to antimicrobial agents, and their propensity to both acquire and to donate novel resistance genes by horizontal transfer. Enterococci are commonly isolated from biofilms formed on implanted medical devices such as catheters and feeding tubes, and colonization and growth on heart valves may represent a form of the biofilm growth state. A recent report suggests the biofilm environment as a likely niche for transfer of high-level vancomycin resistance from enterococci to Staphylococcus aureus. In this project, the physiology and genetics of biofilm formation in E. faecalis will be analyzed using both in vitro and animal models. This work could lead to the identification of novel gene products required for the adaptation of the organisms from a commensal lifestyle to that of a pathogen. The specific aims of the project are: 1) To identify the E. faecalis genetic determinants required for biofilm formation. 2) To use reporter gene constructs to monitor expression in biofilms, and to characterize the sensing machinery used by E. faecalis to adapt to biofilm growth from planktonic growth. 3) To analyze the cellular and molecular structure of E. faecalis biofilms using high resolution scanning electron microscopy. 4) To test the virulence of selected biofilm-deficient mutants in experimental endocarditis.